MAGNITUDES AND RELATIVE ORIENTATIONS OF CHEMICAL SHIELDING, DIPOLAR AND J-COUPLING TENSORS FOR ISOLATED P-31-P-31 SPIN PAIRS DETERMINED BY ITERATIVE FITTING OF P-31 MAS NMR-SPECTRA

Accurate measurements of the magnitudes and relative orientations of c hemical shielding, J coupling and dipolar coupling tensors for isolate d homonuclear P-31-P-31 spin pairs, using standard P-31 MAS NMR experi ments in conjunction with efficient numerical simulation and iterative fitting procedures are described. For a series of transition-metal ph osphine complexes (R3P)(2)MX2, containing pairs of crystallographicall y equivalent phosphorus atoms such that they constitute isolated magne tically nonequivalent P-31-P-31 spin-pair systems, it is demonstrated that the combined effects from MAS averaging, n = 0 rotational resonan ce dipolar recoupling, and J recoupling lead to P-31 MAS NMR spectra w ith detailed structural features. These features are highly dependent on the tensorial interactions as well as on the applied external magne tic field strength and MAS frequency. The experimental observations ar e reproduced by numerically exact computer simulations. Iterative fitt ing of P-31 MAS spectra, recorded under various experimental condition s, allows for determination of accurate parameters for the isotropic a nd anisotropic interactions characterizing the P-31-P-31 spin pair. In particular, we demonstrate that the orientation of the chemical shiel ding tensors relative to each other and relative to the internuclear a xis can be determined with high accuracy.